The calorimetric measurement of reagent strips or films is an important analytical tool in the fields of clinical chemistry and analytical medicine. Colorimetric optical measurements are often used to measure the concentration of chemical analytes. The known colorimetric optical analysis systems usually includes a continuous light source, a sample, and an optical detector aligned such that the light from the light source shines with approximately even intensity onto the sample and is then reflected as evenly as possible onto the detector. In a typical known colorimetric optical analysis system, the continuous light source is an incandescent bulb or a fluorescent tube. The known colorimetric measurement systems thus have mobility limited by the bulk, weight, and geometry of their components.
Optical analyses involving the measurement of light reflected from a flat diffusing surface are subject to Lambert's Law. Lambert's Law states that the luminous intensity in a given direction radiated or reflected by a perfectly diffusing plane surface varies as the cosine of the angle between that direction and the normal to the surface (cos.THETA.'). (See FIG. 7) The efficiency of any optical detector depends upon its projected area and varies as the cosine of the angle of incidence between the normal to the surface and the incident ray (cos.THETA.). Therefore, in situations where the light source shines onto a finite planar surface and where the reflected light is then measured by a detector situated coplanar to the light source, .THETA.=.THETA.' and the decreased intensity from the edges as a function of .THETA. may be expressed as I(.THETA.)=I.sub.o cos.sup.2 .THETA., where I.sub.o is the intensity of the light source.
The concentration of an analyte on a reagent strip sample may be calculated by measuring the optical reflectance from an area of color development on the strip. Whenever the area of color development is non-uniform, or if it consists of a series of dark narrow lines on a brighter background, an optical system capable of resolving the non-uniformity and/or narrow lines must be used. For systems having non-uniform color development, multiple photodiodes can provide some improvement in error associated with the optical non-uniformity. When the color development consists of narrow lines, a distributed detector or a motor with a lensed photodiode has been used to compensate. In many cases, the size of the developing area may be considerable, especially in the case of lateral flow strips where a series of narrow lines develops over a length of up to 25 mm or more. Because the intensity of the signal reflected from the edges of the sample becomes attenuated, the signal-to-noise ratio of the prior art systems decreases below acceptable levels near the edges of the area of interest. This edge attenuation has been compensated for by aperturing a continuous light source such that more light is incident on the edges of the illuminated region. Because such a system requires a continuous light source, such as a fluorescent tube, it is of limited value when the sample is small.
Furthermore, known optical reflectance measurement systems usually require a motor to move the sample under the optics or to move the optics over the irregularly patterned sample. A motor and its required electronic and mechanical support adds bulk, cost, and complexity to the system.
Some known colorimetric analysis systems use one or more discrete light sources, such as light emitting diodes (LEDs) instead of using a continuous light source. However, known systems using a plurality of LEDs as their light source group them in arrays that approximate the light emitted from a continuous source. Furthermore, LEDs cannot be apertured to increase their effective edge illumination intensities. All of these known calorimetric analysis systems employ Lambertian light sources that illuminate the sample less intensely at the edges. Hence, there is a need for an improved optical analysis device for preferentially illuminating the edges of planar samples without resorting to the addition of a motor. A means for satisfying this need has so far eluded those skilled in the art.